This project utilizes newly developed pulsed radiation techniques to examine the kinetics and nature of early important events which are created in the living cell immediately after physical absorption of ionizing radiation and which are believed to constitute important components in the mechanism chain through which cells are lethally injured. The basic approach involves the compression of all of the radiation dose delivered to cells into a single pulse of time duration short in comparison with that of the events under study followed by biological assay of cell viability as various parameters are varied. Using this method the project has succeeded in measuring the time-diffusion of oxygen into mammalian cells, in establishing an upper limit to the lifetime of oxygen-sensitive species produced in cells which are strongly suspected to be involved intimately in the still unknown mechanism through which oxygen sensitizes cells, and in demonstrating that repair of sublethal damage does occur in cells exposed to the ultrahigh dose rates associated with pulsed radiation. Preliminary experiments have also been done to study the kinetics, mechanism and site of damage associated with the action of chemical radiosensitizers which preferentially sensitive hypoxic cells. The project now plans to expand these studies on chemical sensitizers along two lines: (1) using ultrahigh dose rate radiation to examine the kinetics of radiosensitizers relative to those of oxygen; (2) using conventional dose rate radiation to determine the rules by which oxygen and chemical radiosensitizers operate when they must compete for the same short-lived species radiation-induced at the same critical sites within the cell. These latter experiments are believed to be directly relevant to the consideration of hypoxic cell sensitizers for use in radiation therapy. It is also planned to utilize the already developed double pulse technique on phage-infected bacteria to examine the kinetics of oxygen-dependent damage produced at a known site of damage, namely, phage DNA. It is believed that the pulsed experiments proposed will provide new kinetic information important to understanding mechanisms of action through which oxygen and chemical compounds act to radiosensitize cells in the radiation therapy.